The present invention relates to a method and an apparatus for carrying out zero point corrections of temperature dependent characteristics of output signal signals of sensors.
Various types of sensors have their varying temperature dependent characteristics due to environmental temperatures therearound and piece-to-piece variations. Such a sensor has complex temperature dependent factors so that the temperature dependent characteristic of the sensor is usually different from a simple linear characteristic.
In order to correct the temperature dependent characteristic of the sensor, the environmental temperature around the sensor is measured as a temperature parameter signal, and the absolute level of the sensor signal outputted from the sensor, which includes an error depending on the environmental temperature, is corrected based on the measured temperature parameter signal. This results in that the slope of the sensor signal with respect to the environmental temperature is simple linearly corrected.
This absolute correction, however, fails to accurately correct the zero point correction, in other words, offset correction, of the temperature dependent characteristic of the sensor.
Then, as an example of the zero point correction of a temperature dependent characteristic of a sensor, a signal processing circuit that carries out the zero point correction of a temperature dependent characteristic of an oscillation gyro (a yaw rate sensor) is disclosed in Japanese Patent Publication H6-160100.
FIG. 7 illustrates the schematic structure of the disclosed signal processing circuit. In the signal processing circuit, a yaw rate signal obtained by an oscillation gyro 1 is amplified by an amplifier 2, and the amplified signal is synchronously detected by a synchronous detection circuit 3. The synchronous detected signal is quantized (digitized) by an analog-to-digital (A/D) converter 4 as digital data, and the digital data is smoothed by a microcomputer 5.
On the other hand, an environmental temperature around the oscillation gyro 1 is detected by a temperature sensor 6, and a temperature parameter signal based on the detected environmental temperature is inputted to the microcomputer 5 as temperature data.
The microcomputer 5 corrects the smoothed digital data based on the temperature data to obtain temperature characteristic data, storing the obtained temperature characteristic data on an EEPROM (Electrically Erasable and Programmable Memory) 7.
When measuring a yaw rate of an object by the oscillation gyro 1, the smoothed digital data corresponding to the yaw rate of the object detected by the oscillation gyro 1 is inputted to the microcomputer 5 through the amplifier 2, the synchronous detection circuit 3, and the A/D converter 4.
The microcomputer 5 reads out the temperature characteristic data from the EEPROM 7 to carry out the zero point correction of the digital data based on the read-out temperature characteristic data. A digital-to-analog converter (D/A) converter 8 converts the zero-point corrected digital data into analog data to output the analog data as the zero-point corrected yaw rate of the object. The above signal processing allows the zero point correction to be effectively executed.
The disclosed signal processing circuit, however, requires the microcomputer 5 for performing the zero point correction, causing the cost of the signal processing circuit to rise.
In addition, the disclosed signal processing circuit requires the A/D converter 4 for converting the yaw rate signal into the digital data, and the D/A converter 5 for converting the zero-point corrected digital data into the analog data corresponding to the zero-point corrected yaw rate of the object.
The analog-to-digital conversion processing (quantization processing) of the yaw rate signal itself, and the digital-to-analog conversion processing of the zero-point corrected digital data may increase the total processing time of the zero point correction processing, and, especially, quantization errors may be included in the zero-point corrected yaw rate of the object.